Submitted to: North Central Extension Industry Soil Fertility Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 11/19/2003
Publication Date: 11/19/2003
Citation: DAVIS, J.G., SCHARF, P.C., KITCHEN, N.R. SITE-SPECIFIC NITROGEN MANAGEMENT FOR REDUCING SOIL RESIDUAL NITRATE. PROCEEDINGS NORTH CENTRAL EXTENSION INDUSTRY SOIL FERTILITY CONFERENCE. 2003. P. 76-86. Interpretive Summary: Higher fossil fuel prices have recently caused nitrogen (N) fertilizer costs to nearly double. This, along with increasing environmental concern for how agricultural use of N is affecting life in streams, rivers, and the ocean, has motivated farmers to consider how N fertilizers can be more efficiently managed. Precision farming (sometimes called site-specific farming) methods which adjust inputs to match crop needs is one proposed way of more efficiently managing N. This study was conducted on farmers' corn fields to assess how much N remained in the soil after harvest when varying N fertilizer in the spring. Nitrogen fertilizer was applied at many different rates in strips over fields that varied in soil types. The amount of N that was needed to grow corn was found to be highly variable over the field. Within some fields the amount needed varied from 50 to over 200 lbs of N per acre. As expected, where N was applied at amounts greater than what the corn crop needed (i.e., an excess), then we found with our soil samples that N accumulated as soil nitrate. This is a concern because soil nitrate at the end of a growing season is prone to leaching into ground water and then into streams. Soil sensors were also used on these research fields to evaluate soil texture (e.g., amount of sand, silt, and clay) as it varied through the field. We were not able to show a consistent relationship between these soil sensor measurements and the amount of nitrate in the soil. Careful characterization of field variability is needed for developing site-specific plans that are more efficient and environmentally friendly. The results of this study will be used to evaluate the environmental benefit of different approaches for applying N fertilizers.
Technical Abstract: Site-specific N management has potential for increasing the efficiency of N fertilizer use and thereby reducing environmental impact. Field studies were conducted in 2000 and 2001 to evaluate the potential for site-specific N management to reduce residual soil nitrate in Missouri cornfields. Field-size side-by-side fertilizer N treatment strips were applied in cooperating farmers' fields. The site-specific crop N requirement (optimal N rate) was determined for sub-blocks that contained a full set of N rate treatments. Residual soil nitrate content was determined for selected N-rate sub-blocks to investigate the relationship between residual soil nitrate and optimal N rates. Prior to establishing the N treatments, apparent bulk profile soil electrical conductivity (ECa) measurements were collected for each site. Soil sample locations for residual soil nitrate were selected based on the observed contrast in ECa within each field. Wide variations in the site-specific N requirement for corn were observed. We found that residual soil nitrate (RSN) levels were greater for higher fertilizer N rates and for late-season applications. Most of the RSN was in the surface 30 cm soil layer, except for the highest N rate(280 kg N/ha). We were not able to confirm a consistent relationship between ECa and residual soil nitrate. Residual soil nitrate accumulated in soils where the amount of applied N exceeded the optimal N rate. Additional analyses will be done to examine the spatial relationship in RSN levels within these fields. The results of this study will be used to evaluate the environmental benefit of developed variable-rate N strategies.